When selecting metal tubing for a particular application, the demands of the application may require physical characteristics which cannot be met by a single composition. Some alloys, for example, may provide suitable characteristics in terms of mechanical properties but lack the necessary resistance to corrosion, abrasion, or both. Other alloys may provide suitable performance in resisting corrosion and abrasion but lack the requisite structural characteristics.
In such an instance, the solution is normally found in overlaying a tubing having the requisite structural characteristics with an alloy having the desired resistance to corrosion and abrasion. Such composite tubing is typically manufactured in either of two ways: by co-extruding the two compositions during the manufacturing process or by applying a weld overlay to a pre-existing metal tube. The present invention is concerned with the latter approach.
An optimal weld overlay has several characteristics. First, the weld penetration of the tube stock is carefully controlled to ensure the optimum fusion between the weld overlay and the tube stock. Second, the exterior surface of the weld overlay will be smooth. Third, the thickness of the weld overlay and the resulting dimensions of the overlayed tube can be closely controlled. Unfortunately, using conventional weld overlay techniques, these characteristics tend to be mutually exclusive, and the weld overlay is at best a compromise. If the voltage and current of the weld head are controlled to provide the smoothest possible exterior surface, then penetration of the tube stock tends to be excessive and uneven. Stresses resulting from bending of the tube or from thermal cycling can cause the weld overlay to separate from the underlying tube stock. On the other hand, if the voltage and current of the weld head are controlled to optimize penetration, then the exterior surface of the weld overlay tends to be rough and uneven. A rough, uneven exterior surface of a weld overlay suffers numerous disadvantages. It can make the tube more difficult to bend, which can result in an uneven distribution of stresses during bending and thermal cycling. It can also affect the thickness of the weld overlay and the resulting dimensions of the overlayed tube. When the tubes are used to fabricate a tube panel, adjacent tubes with uneven surfaces can create undesirable gaps between the tubes.
In addition, conventional weld overlay processes create what is known as a heat-affected zone, that is, a zone within the pipe which is created by the metal of the pipe being heated to a temperature higher than its Ae1 temperature but less than its melting temperature. The base metal within the heat affected zone undergoes phase, microstructure, and grain growth changes which can cause the pipe to be brittle, such that the composite pipe is difficult to bend without cracking. Furthermore, this transformation results in residual stresses being formed within the pipe.
Thus, there is a need for a weld overlay process and article of manufacture which provides optimum penetration of the underlying tube stock and a smooth, even exterior surface.
There is a further need for a weld overlay process and article of manufacture which can provide optimum penetration of the underlying tube stock and a smooth, even exterior surface and also control over the thickness of the overlay and the resulting dimensions of the overlayed tube.
There is still a further need for a weld overlay process and article of manufacture which can provide optimum penetration of the underlying tube stock and a smooth, even exterior surface and also provide uniform residual stresses within the overlayed tube.
There is yet another need for an overlayed tube which exhibits improved metallurgical characteristics of the base metal within the HAZ